1. Field of the Invention
The present invention relates to a type of semiconductor integrated circuits (ICs) generally referred as a system-on-chip (SOC). More particularly, the invention relates to a power control circuit for a SOC.
2. Description of the Related Art
As contemporary semiconductor ICs are scaled down to nanometer sized dimensions, difficulties with power leakage for constituent circuits increase. The incorporation of such ICs into portable electronic devices generally running from batteries power supplies only increases the demand for very low loss power supply systems and circuits. Many different approaches have been taken to the reduction of current leakage in such systems and circuits. The use of various control methods, power gating schemes, and retention flip-flops are ready examples of these approaches.
Power gating schemes are generally useful in the reduction of current leakage. These schemes interrupt the application of power to circuit blocks current unused in a SOC. However, where volatile memory devices are incorporated into circuit blocks, the interruption of power will cause a loss of stored data. Thus, in order to retain vital data, it must be backed up prior to power interruption, and thereafter recovered upon re-application of power. Unfortunately, data backup and recovery operations tend to markedly slow the overall operation of a SOC.
In order to avoid burdensome data backup and recovery operations, retention flip-flops have been used in certain designs. As their name suggests, retention flip-flops retain stored data without the application of power. This capability is enabled by very small amount of leakage current associated with retention flip-flops. While the use of retention flip-flops allows data backup and recovery operations to be dispensed with, retention flip-flops also occupy a larger amount of IC surface area, as compared with conventional flip-flops. Thus, as the number of retention flip-flops incorporated into a system design increases, the overall IC chip size tends to undesirably increase.
Therefore, an alternate power control approach is required that allows retention of stored data without unduly expanding the size of a SOC.